Silicate materials with designed porosity are useful for a number of applications in the chemical, biological, optical, and electronics industries. Of particular interest are the mesoporous silicates, as described, for example, by A-S. Malik et al, Mat. Res. Soc. Symp. Proc., Vol. 658, Materials Research Society, 2001 (GG7.51-7.5.5). The mesoporous materials can be formed into optically transparent, monolithic structures. Formation is sometimes based on the “surfactant L3 phase”. Such a phase is known in the art, and is often prepared by combining water, hexanol, and a surfactant such as cetylpyridinium chloride (CPC).
One of the first descriptions of the synthesis of a continuous, mesoporous silicate structure is provided by K. M. McGrath et al, “Science”, Vol. 277, Jul. 25, 1997, pp. 552-555. In that preparation, the lyotropic surfactant L3 phase was used as a template for the silicate L3 phase. The resulting structure had a variety of unique characteristics, such as optical isotropy, adjustable pore size, and uniform pore structure. Moreover, the structure had a very high surface area, and the pores could be accessed from many points on the surface.
The mesoporous structure described in the McGrath article includes a network of interconnected pores. The walls of the pores are formed of the silicate material, and they divide an associated liquid medium (e.g., water) into two volume fractions. The overall composite structure provides potential locations for functionalization with a variety of reactive groups. As an example, functionalization can occur within the interconnected pore network, or between the silicate layers which constitute the walls of the pores.
The Malik article provides a good example of one type of functionalization. In that illustration, a mesoporous silicate structure is “impregnated” with photopolymerizable monomers. Malik et al report that a laser could be used to selectively polymerize the monomers, so as to potentially create patterns on the composite. In this manner, unique opto-electronic properties could be obtained. The article also alludes to the frequent difficulty in forming mesoporous structures which are physically stable, and which have the strength to accommodate practical end uses.
Still, functionalization of the “scaffold” of the mesoporous silicate layer can potentially open up many new opportunities for these materials. Some of the possible applications are described in the articles mentioned above, but future work could greatly expand the opportunities. As an example, a functionalized mesoporous network structure could serve as the electrolyte for any type of device that may require a continuous path of electrical conductivity, e.g., a fuel cell.
As another example, a mesoporous structure of this type could be used as a biomolecular sensor. The ability to control the size of the mesopores would allow one to “tune” the sensor to accommodate selected molecules of the biological agent. As yet another potential application, these mesoporous structures could be used as filters for various separation processes, e.g., removal of heavy metals or other contaminants from the environment.
In many of these potential uses for mesoporous structures, it is the functionalization itself which may be the greatest factor in enhancing the application. As an illustration in the case of the biosensors, the mesopores in the structure may provide an initial “screening” for biological agents. However, much greater precision in screening could subsequently be obtained by incorporating a number of functional groups into the structure. As one example, the groups could be selected according to chemical compatibility or incompatibility with the agents, or with any “markers” associated with the agents.
It should thus be apparent that there is considerable interest in finding new ways to functionalize mesoporous silicate structures. The new processes should be capable of providing selected, functional sites in a number of locations within the structural network. Moreover, the functionalization processes should be generally compatible with available techniques for making the structures. For example, they should not add excessive cost to the overall manufacturing operations. Furthermore, it would be helpful if the new processes generally maintained the physical integrity of the mesoporous structure.